1. Field of the Invention
The invention relates to correction of raster distortion on the display screen of a cathode ray picture tube (CRT), and particularly to a circuit for correcting such distortion without requiring modification of the CRT deflection yoke for that purpose. Such correction is particularly important in a projection television receiver, wherein the respective images on the display screens of three monochrome CRTs are projected to form a composite color image on a projection screen.
2. Description of the Related Art
Projection television receivers commonly employ three monochromatic CRTs producing respective primary color components of a composite color video image, the monochrome images being combined by projection so as to form the composite color image on a projection screen.
It is, of course, essential that the monochrome images be precisely superposed at the projection screen, which condition is preferred to as image convergence. Convergence error, or misconvergence, is frequently caused by non-linear distortion of the raster on the display screen of each of the CRTs. Such distortion frequently takes the form of "pincushioning", due to an excessive increase in the deflection of the electron beam of the CRT near the edges of the raster.
It is known to provide convergence correction by providing compensating non-linear distortion of the electron beam deflection. That has been achieved by providing either a non-linear deflection yoke design including additional convergence correction windings, or by providing non-linear waveforms of the sweep signals which generate the raster. See, for example, the text book "Television Engineering Handbook", K. Benson, McGraw-Hill Book Co. 1986, par. 10.5.2. However, such methods due not produce entirely satisfactory results and are expensive to implement. It would be advantageous to provide correction for image raster distortion without altering the linear design of the deflection yoke or the linearity of the sweep signals which produce the raster on the CRT display screen.